There are many applications for positioning systems having accurate controllable motor drives. One common application is in X-Y plotters. The typical plotter employs an X-axis drive mechanism for the chart or like media, and a Y-axis drive for the pen, the combination of movements allowing the plotter to produce virtually any kind of line drawing on the chart.
A desirable condition for the drive mechanisms in such apparatus is that there should be minimum slippage and backlash in the system. This is required because the execution of a line drawing by the plotter requires that the chart continuously move back and forth in the X-direction, and it is important that the X-axis drive mechanism be capable of repeatedly positioning the chart to very close tolerances at the selected positions along the X-axis. The same requirement, of course, exists for the Y-axis drive mechanism for the pen.
These problems are exacerbated when a synchronous belt-pulley system is employed in the drive mechanism, rather than spur gears. The former system is preferred mainly because it reduces the cost of the drive mechanism, and it somewhat simplifies the location of parts in a housing that historically has become more miniaturized. But, to satisfy the requirements indicated above with a belt-pulley drive system requires a mechanism not only for providing the proper tension on the belt, but also for maintaining the belt tension at the correct value despite pulley eccentricity and other variables during the lifetime of the product.
In one known system, a spring-loaded, cam-driven, idler roller operates against the outer surface of the belt producing a reverse wrap which provides the required tension. However, this arrangement applies tension and compression loads to the belt for which it was not designed.